This invention relates to electronic test and measurement systems for precisely measuring the performance characteristics of unknown devices under test and, more particularly, to techniques for assuring, with a relatively high probability, that such systems are able to perform calibrated performance measurements on test devices. Specifically, one embodiment of the invention is directed to a method for verifying that a test and measurement system continues to operate within a predetermined acceptable range with respect to a given calibrated state previously defined for that system. This method can be applied to verification of test and measurement systems for performing electrical, as well as electro-optical, opto-electrical, and optical, performance measurements on a device under test.
Factory calibration of a radio-frequency (RF)/microwave test and measurement system, or network analyzer, for measuring the performance characteristics of an unknown device under test assures system integrity, such as receiver linearity, frequency accuracy, and measurement dynamic range, at the time of manufacture. The network analyzer can also be recalibrated when it is later maintained or repaired.
Additionally, a conventional network analyzer is typically subjected to a measurement calibration process just prior to the time that a user performs a measurement on an unknown device under test. The calibration measurement involves measuring various calibration standards. These calibration standards comprise reference components, such as a short-circuit, an open-circuit, a known termination (e.g., a load), etc., whose electrical properties have been previously measured on a calibrated network analyzer and whose performance characteristics are therefore precisely known. These calibration standards are sequentially connected to the actual network analyzer being used and are then measured, and any differences between the calibration measurement data and the expected measurement data are used as error correction data when the performance characteristics of an unknown device under test are subsequently measured. However, the quality of the measurement performed by the user is not only dependent on the quality of the reference components of the calibration kit and the quality of the user's measurement procedures, but also is dependent on the network analyzer hardware performance.
Consequently, in order to verify the performance of network analyzer measurements, after the foregoing calibration measurement process is completed, a verification kit is typically used. The verification kit comprises a set of reference components and, additionally, accompanying performance characterization data for these components, that are previously measured on a calibrated network analyzer. Typically, the reference components incorporated into the verification kit comprise highly precise components similar to the calibration standards used to calibrate a network analyzer. Comparison of the accompanying performance characterization data with data later measured by the actual network analyzer being used after undergoing a calibration measurement is employed to predict (with reasonable probability) the integrity of subsequent network analyzer measurements on unknown devices under test.
However, manufacture and characterization of the reference components of verification kits is difficult. Moreover, the cost of verification kits is relatively high. Furthermore, the user is required to not only purchase such a costly verification kit, but also to maintain it and assure that it remains with the network analyzer so that the additional verification process can be performed.
Furthermore, unlike the case of conventional electrical measurements, the concept of verification for electrical/optical measurements poses a significant additional challenge. The availability of representative devices to use as calibration or verification kit reference components is uncertain. In any event, to provide a complete verification would be extremely costly, and probably prohibitively expensive, in the case of an electrical/optical network analyzer, or lightwave component analyzer. However, without a calibration or verification kit being available to provide a traceable measurement solution, it is not possible for the user to gain confidence in the integrity of lightwave component analyzer performance measurements on unknown devices under test.